It is useful in many applications to detect ionizing radiation. Applications for detecting ionizing radiation include, but are not limited to imaging, nuclear detection, particle physics, gamma ray astronomy, nuclear non-proliferation and homeland defense.
As one nonlimiting example application, positron emission tomography (PET) is a non-invasive imaging technology that enables visualization and quantification of the molecular signatures of disease in living subjects in a clinic as well as in animal research. In clinical use, PET is standard-of-care for diagnosis, staging, and monitoring treatment for many types of cancers. The method involves injection of a trace amount of a radioactively-labeled chemical, referred to as the molecular probe, into the subject. The label is a type of radionuclide that emits positrons. Ideally, the probe is taken up in the cells of tissues in proportion to the presence of the molecular signature of disease. For clinical management of cancer, the most common probe used is a radioactive analog of glucose, known as 18F-fluorodeoxyflucose (FDG), and the cancer signature is an up-regulation of cellular glycolysis.
In an example imaging method the patient is placed in a PET scanner, comprising a ring or cylinder arrangement or various other arrangements of high-energy photon detector elements, typically photon sensors known as scintillation crystal detectors (scintillation detectors), and supporting electronics, which makes a quantitative image of the probe biodistribution. The radionuclide attached to the probe molecule emits a positron, which subsequently annihilates with a nearby electron, yielding two oppositely directed photons, each having energy of 511 kilo-electron-Volt (keV), the rest mass energy of the electron (or positron). The two annihilation photons are emitted from the body and detected nearly simultaneously in two small opposing detector elements that localize each photon's point of entry into the system as well as measure their energy and arrival time. If two photons on opposite sides of the system arrive within a selected coincidence time window setting, it is assumed that they came from the same positron decay event. This coincidence detection process localizes each positron decay event, and thus the molecule of interest, to somewhere along the line of response (LOR) extending between two detector elements. Successively detected coincident photon events are aggregated throughout the PET system over a period of time, for instance ˜30 minutes. Then, mathematical statistical likelihood algorithms, typically based on the framework of maximum likelihood estimation maximization (MLEM), are used to reconstruct three-dimensional (3-D) images of the probe biodistribution that most likely created the pattern of many two-photon hits recorded by the scanner.
These algorithms involve computationally intensive back and forward line-projection operations along each LOR through the 3-D image volume. Further, conventional PET can exhibit less than desirable signal-to-noise ratio (SNR) in the reconstructed images.
Significant research has focused on improving the scintillation crystal of the detector to go beyond simple coincidence detection capability to time-of-flight (ToF) capability. In ToF-PET, in addition to being able to determine to which detector LOR the event belongs, the detector coincidence time resolution is high enough to enable one to constrain the two photon emission point to within a particular segment along that line. However, in known ToF PET systems, timing resolutions tend to drift with time and suffer from count rate limitations. While improved ToF-PET systems have demonstrated improved lesion contrast and image SNR for larger patients, little or no image SNR benefit is provided for smaller patients or for small animal imaging.